368 PROCEEDINGS OF THE AMERICAN ACADEMY. 



fusion. If mercury is heated under similar conditions it is impossible to 

 obtain shifts of more than a fringe or two. Potassium distils almost in a 

 flash to the colder parts of the tube. The other alkali metals have not as 

 yet been investigated. The vapor of sodium appears to possess a cohe- 

 sion almost as great as that of a liquid. Even in a vacuum tube it ap- 

 pears to have a free surface, while in an atmosphere of hydrogen it can 

 be dipped up on a wire, a vigorous shaking of which is insufficient to 

 dislodge it or cause its dissipation. A number of very interesting exper- 

 iments with the vapor have already been made, and others are in progress 

 at the present time. They will be reported in a subsequent paper ; as for 

 the present we are only concerned with the fact that a homogeneous 

 cylinder of very dense vapor terminated with sharply defined ends can 

 be formed in an exhausted tube. 



FniUKE 2. 



Anomalous dispersion occurs at the absorption lines which belong to 

 the principal series, being very strong at the D lines, feeble at the first 

 pair of ultra-violet lines (A = 3303), and almost imperceptible at the 

 second pair (X. = 2852), as is shown in Figure 2. If the vapor is of 

 considerable density, the D lines run together into a single absorption 

 band, and the dispersion can be represented throughout the complete 

 range of wave-lengths as resulting from electrons of a single period, 

 for the ultra-violet bands only affect the refractive index in their imme- 

 diate vicinity. These bands make their appearance in succession as 

 the density of the vapor increases, the second not appearing until the 

 band at the D lines has become three or four times as broad as 

 the distance between the lines. The complicated channelled absorp- 

 tion spectrum appears to exert no influence on the refractivity of the 

 vapor. 



I feel certain, however, that the channelled absorption spectrum affects 

 the refractivity of the medium in a small degree, and intend to test the 



